The open-end spinning process consists essentially of the following stages:
feeding the fibre sliver to the spinning station by a feed roller; PA1 separating the sliver by a toothed carder which rotates at high speed and separates the sliver into individual fibres; PA1 pneumatically feeding the individual fibres to the hollow spinning rotor which is provided with an inner groove within which the fibres are deposited in layers by the effect of the centrifugal force generated by the rotor rotating at very high speed; PA1 an already formed yarn is initially inserted through the a channel located substantially on the axis of rotation of the rotor, centrifugal force propelling its free end to the periphery, i.e. into the groove where it encounters the fibre layer; on drawing out the yarn the fibres become joined to the yarn, acquiring twist in the section between the groove and the exit channel to produce new yarn.
In the known art the spinning machine is provided with yarn-feeling sensors which for every yarn breakage cause the fibre sliver feed roller to stop, as for example, in Italian Patent No. 791,993 of VUB.
However the separating carder continues to rotate even if yarn production is interrupted.
In the known art the method used for rejoining the yarn and restarting production comprises firstly cleaning the spinning rotor, in which irregularities or dirt build-up have probably occurred.
This cleaning is done either by opening the rotor, clamping it and then using suitable tools such as brushes, suction nozzles, spatulas etc., or by keeping the rotor closed and using an air blast.
After cleaning, a fibre layer suitable for forming new yarn is rebuilt in the spinning rotor while under movement, and the interrupted yarn end is reinserted when the rotor has reached suitable speed, to "fish out" this fibre layer and again produce yarn.
This operation is conducted either manually, in non-automated spinning machines, or automatically by devices located for example on mobile carriages which patrol the plurality of spinning stations along the machine face in the case of automated machines, as described in U.S. Pat. Nos. 3,810,352 and 3,950,926.
A technical problem common to both automated and non-automated spinning machines is that the fibre feed sliver which has been halted on yarn breakage, with its feed roller at rest, remains with its end exposed to the action of the separating carder. Generally, the rejoining operation is commenced with variable delay because the rejoining operation requires in the case of non-automated spinning machines the operator, or in the case of automated spinning machines the carriage, to arrive in front of the spinning station at which the operation is to be carried out. The operator or carriage can be a variable distance away, or may be engaged in other operations.
During this variable waiting time the end of the feed sliver continues to undergo separation, without advancing, and is depleted of fibres, which are gradually removed by the carder teeth.
When the sliver is reused in that state for restarting the spinning, the fibre layer newly deposited in the rotor groove and used for rejoining purposes gives rise to thinner or fatter weakened portions and thus an irregular joint, with the result that the produced yarn is of poor quality .